1. Biochemical organization of several retroviral onc genes was studied by nucleotide sequence analysis. The transforming genes of BALB-MSV, Abelson-MuLV, SSV, MC-29 and AMV have been sequenced. 2. Several deletion mutants were constructed using the proviral genome of A-MuLV to understand the structural requirements for transforming gene function and to delineate the regulatory signals present in retroviral long terminal repeats. 3. Using cloned proviral DNAs of Rauscher and Moloney murine leukemia viruses, several recombinants were constructed between the two viruses to map the regions in the leukemia viral genome responsible for target cell specificity.